Is there any phrase more meaningless, empty or evasive than that notorious alibi of inadequate screenwriters and undernourished scripts, "Based on a true story ..."?

Usually what it means is that, as Marge once put it in The Simpsons when one or another of Bart's outrages was turned into a TV movie, "They've changed the facts just enough that they don't have to pay us a dime."

At other times it can mean that the film-makers spotted an item in a newspaper, worked it up into a harrowingly awful script, and then found that the only redeeming merit of the resulting celluloid catastrophe was that it had some tenuous foundation in reality.

It also serves as a useful smokescreen for rotten movies that take actual events and superimpose upon them the kind of witless love stories, "narrative clarifications and simplifications" and outright distortions that, in the eyes of brain-dead studio executives, really make a movie. You know the drill: "Yeah sure - we get the concentration camp stuff and the whole genocide-is-terrible angle, baby, but can't we work a cute cheerleader role in there somewhere?"

Rob Reiner's 1996 film Ghosts Of Mississippi played out a lot like that. Somehow, Reiner managed to make the FBI the heroes of the civil rights movement, when in reality the organisation was almost as big a problem as southern bigots. The people who really put their lives on the line for civil rights - black folks who were quite right to fear murder and lynching at the drop of a hat - were reduced to silent, superstitious dummies being freed from oppression by big whitey.

Based on a true story, sure, but also based in a completely parallel universe of delusion and revisionism.

Or how about The Exorcism Of Emily Rose? This is, shall we say, "loosely" based on the story of an Austrian student who died in 1974 during an exorcism, apparently of starvation rather than demonic possession. The exorcists were tried and found guilty of negligent homicide in a court of law, where God, quite sensibly, is treated as a bullshit abstraction with no grounding in reality.

Given the fact that this entire event occurred during the sensational, yearlong and worldwide success of William Friedkin's The Exorcist, it might plausibly be argued that, far from being based on a true story, this incident was based on ... The Exorcist. If only more "based in truth" movies were as honest as Henry: Portrait Of A Serial Killer. It took a kernel of dramatic possibility from the confessions of Henry Lee Lucas (he claimed he'd offed 300 people, but probably only managed about 12), and delivered a chilling drama about aimless murder in America's lumpen drifter class. And it didn't bother trying to impress us with any verifiable "truth". Because truth is often very boring indeed.